The Road from Stalingrad: Joel Hayward, "Hitler's Quest for Oil: the Impact of Economic Considerations on Military Strategy, 1941-42" http://joelhayward.org/Hitlers-Quest-Finished.pdf:
[O]n 28 June  the main campaign to seize the Caucasus oilfields had been launched and initially, to Hitler's great delight, made startling progress. The Volga river north of Stalingrad was reached by soldiers of the Northern Army Group on 23 August, allowing them to sever the Soviet's main north-south supply and communication route. Stalingrad, not as yet a main objective, seemed certain to fall within a month. On 9 August, German troops of the Southern Army Group had even overrun Maikop…. By late October, Hitler seemed very close to victory. However, his Southern Army Group's attempts to push past Pyatigorsk and secure the passes through the main Caucausus range to the far richer oilfields in the south were repeatedly thwarted….
[T]he disastrous campaign was, from its very conception, the result of perceived economic necessity. Its planning was, to a far greater degree than that of any other German military undertaking of the Second World War, significantly influenced by economic considerations. The Führers directive of 5 April 1942 committed the German armed forces for the first time to a massive offensive with economic objectives taking precedence over strictly military ones. Hitler was well aware that… he commanded mechanized armies in a war between major industrialized nations. He knew that the economic resources (including manpower, raw materials and fuel) of the growing list of nations he now faced greatly exceeded Germany’s…. Unless he succeeded in both defending his main existing source of oil from Soviet air attack and in capturing new and substantial sources he was incapable of waging a protracted war of economic attrition. On the other hand, he believed, if his campaign succeeded it would not only relieve the terrible shortage of oil products currently experienced but also deliver a massive, possibly mortal, blow to the Soviet war economy.
It needs to be said, of course, that the dire predictions of Hitler and his economic advisers in 1941 and 1942 about the certain collapse of the German war machine if no new sources of oil were obtained proved to be exaggerated. The German war effort did not grind to a halt when the campaign to capture the Caucasus oilfields failed. Although Germany's oil situation remained acute, and became desperate after the Allied air offensive against its synthetic fuel plants and the Rumanian oilfields began, the Reich continued fighting until May 1945.
In fact, despite the total failure of the 1942 campaign, events in 1943 actually led to a slight improvement in the oil situation…. Germany's synthetic fuel industry, not yet targeted by Allied bombers, reached a production peak… the oil shortages which had bedevilled the Wehrmacht's efforts throughout the previous two years appeared far less critical. The Luftwaffe was even able to build up its meagre reserves slightly for the first time since the beginning of the Russian campaign.
History has exposed the inaccurancy of the warnings the Nazi leader received from his economists, but at that time they seemed both credible and compelling. Hitler, aware by December 1941 that he now faced a prolonged war against the world's two economic giants the Soviet Union and the United States, felt that he had no real option but to embark on a campaign which would, if successful, greatly enhance his ability to continue waging that war…